1. Field of the Invention
The present invention relates to composite materials which can be suitably used as materials included in light-emitting elements. The present invention further relates to light-emitting elements, light-emitting devices, lighting devices, and electronic devices including the composite materials. Furthermore, the present invention relates to fluorene derivatives which can be suitably used for the composite materials.
2. Description of the Related Art
As next generation lighting devices or display devices, light-emitting devices including organic EL elements (light-emitting elements in which organic compounds are used as light-emitting substances) have been developed at an accelerated pace owing to their advantages of thinness, lightweightness, high speed response, low power consumption, and the like.
In an organic EL element, a light-emitting layer including a light-emitting substance is interposed between a pair of electrodes. Application of voltage between the electrodes causes electrons and holes to be injected from the electrodes to recombine and form an excited state. When the excited state relaxes to a ground state, light is emitted. The wave length of light emitted from a light-emitting substance is peculiar to the light-emitting substance; thus, by using different types of organic compounds as light-emitting substances, light-emitting elements which exhibit various wave lengths, i.e., various colors can be obtained.
In a case of a display device which is expected to display images, such as a display, at least three colors of light, i.e., red, green, and blue are required to be obtained in order to reproduce full-color images. In a case of a lighting device, in order to obtain a high color rendering property, it is ideal to obtain light having wave length components thoroughly in the visible light region. Actually, two or more kinds of light having different wave lengths are mixed to be used for lighting application in many cases. Note that it is known that by mixing light of three colors, red, green, and blue, white light emission having a high color rendering property can be obtained.
Important characteristics of a light-emitting element include emission efficiency in addition to the color of the light-emitting element. Compared with other light-emitting elements, a light-emitting element with high emission efficiency needs lower energy to obtain the same luminance. Therefore, it is possible to provide a light-emitting device having a high energy-saving property. Now that much attention has been paid on the energy issue, increase in emission efficiency is a significant object.
One of useful methods for increasing emission efficiency, particularly power efficiency, is reduction in driving voltage. When the driving voltage becomes lower, a larger current can flow at a low voltage, which results in lower power consumption and higher power efficiency. Thus, many researches have been made aiming at reducing a driving voltage (e.g., see Non-Patent Document 1).
In addition to emission efficiency, important characteristics of a light-emitting element also include a life (reliability) which relates to the credibility as a product. As a matter of course, the life (reliability) is one of characteristics required in commercialization, so that a life (reliability) at a certain level or higher can be a big selling point. That is, the longer the life is (the higher the reliability is), the more preferable. Thus, researches have been made on prolonging the life (increasing the reliability) of a light-emitting element by a variety of methods (e.g., see Patent Document 1).
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 2006-324650[Non-Patent Document]    [Non-Patent Document 1] Y. Yang and A. Heeger, “Polyaniline as a transparent electrode for polymer light-emitting diodes: Lower operating voltage and higher efficiency”, Applied Physics Letters, Vol. 64, No. 10; pp. 1245-1247 (1994).